NSF Award
0927472
The overall objective of the project is to understand better the instabilities of the western boundary currents in the tropical Indian Ocean, which are the Somali Current in the Northern and the East African Coastal Current in the Southern hemisphere. This will in particular allow for determining whether the instabilities of these currents contribute to the generation of Yanai waves, also known as mixed Rossby gravity waves, in the western Indian Ocean. Yanai waves, which have the characteristics of Tropical Instability Waves, have been observed in the Pacific, Atlantic, and Indian Ocean. While the generation mechanism is well understood in the Atlantic and Pacific Ocean, it is still practically unknown in the Indian Ocean. This knowledge gap is the main motivation for the work.<br/><br/>The project will employ the classical normal mode stability theory, which considers exponential growth processes as for instance in the theory of baroclinic instability, and the comparatively new Generalized Stability Theory, which encompasses also transient growth processes. To investigate the instabilities of the western boundary currents in the tropical Indian Ocean three models will be used: A modified form of the quasigeostrophic model, the equatorial counterpart of the quasigeostrophic model recently developed by Theiss and Mohebalhojeh, and the Regional Ocean Modeling System (ROMS).<br/><br/>The overall objective inspires two additional objectives. The first additional objective is to investigate whether the equatorial counterpart of the quasigeostrophic model is for instability studies in the equatorial region a tool that is as instructive as the quasigeostrophic model in the mid-latitudes. The second additional objective is to integrate the project with the PIs' Zanzibar Project, which is an educational project for students on coastal oceanography in East Africa. The natural overlap of the study of the western boundary currents of the tropical Indian Ocean and coastal oceanography in East Africa will be used by the PIs for the mutual benefit of both projects and especially the involved students.<br/><br/>Better understanding the instabilities of the western boundary currents in the tropical Indian Ocean and in particular their contribution to the generation of Yanai waves would help towards developing a level of understanding that already exists about the generation of Yanai waves in the Atlantic and Pacific Ocean. Dominant features such as the alternating Monsoon winds must affect the instabilities of the western boundary currents in the tropical Indian Ocean. These instabilities in turn must affect the marine ecosystems along the East Coast of Africa and might contribute to the generation of Yanai waves, thus effect the momentum and heat balance in the equatorial Indian Ocean and climate variability. The instabilities are therefore an integral part affected by and in turn affecting processes with scales ranging from local to ocean-basin-wide. In addition, the integration of the study with the PIs educational Zanzibar Project will help expand the students' study of the coastal ocean of East Africa to the larger western tropical Indian Ocean.
